Patent Documents 1 and 2 listed below disclose such a hollow poppet valve as described above, in which its internal cavity is charged with a coolant (e.g. metallic sodium which has meting point of 98° C.) together with an inert gas, wherein the coolant has a higher heat conductivity than the valve material.
Since the internal cavity extends in the valve head cavity and stem, it may contain a large amount of coolant, which enhances heat transfer ability (hereinafter referred to as heat reduction effect) of a valve.
If a combustion chamber of an engine is heated to an exceedingly high temperature during an operation, knocking may occur, which lowers the fuel efficiency, and hence the power, of the engine. In order to lower the temperature of the combustion chamber, there has been proposed a method of storing a coolant in an inner cavity of a hollow poppet valve together with an inert gas to positively conduct heat from the combustion chamber via such valve (i.e. a method of enhancing heat reduction effect of the valve) to remove heat from the combustion chamber.